SOx Additive systems based upon use of multiple particle species

ABSTRACT

The useful life of SO X  additives having a SO 2  →SO 3  oxidation catalyst component and a SO 3  absorption component can be extended by employing each of these components as separate and distinct physical particles, pellets, etc.

RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 08/746,837 filed Dec. 11, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to methods and compositions forreducing sulfur levels in flue gases generated by hydrocarbon catalyticcracking units, coal and/or oil-fired power plants and chemicalmanufacturing facilities.

2. Description of the Prior Art

Sulfur is often a component of the feedstocks processed by manyindustrial facilities. It also is found in the fossil fuels used topower and/or create process heat for such facilities. Hence, the sulfurcontained in such materials has the potential to become an atmosphericpollutant--especially when it takes the form of those sulfur oxide gasesthat become a part of the flue gases emitted from such facilities. Suchemissions are particularly harmful to the atmosphere and, hence, are thesubject of extensive governmental regulation. One of the most commonlyused methods for preventing release of these sulfur oxide gases into theatmosphere is to capture them through use of compounds that have anability to absorb them.

For example, in the case of recovering sulfur oxide gases from fluegases generated by fluid catalytic cracking units (FCC units) used tocrack petroleum feedstocks, microspheroidal catalyst particles havingchemical activities toward sulfur oxide gases are circulated inadmixture with the microspherical particles used to carry out thepetroleum cracking function. These hydrocarbon cracking catalystparticles are often referred to as "bulk" or "FCC" catalysts while thesulfur catalyst particles are often referred to as "SO_(X) additives."

During the hydrocarbon cracking process, a coke-like material that alsocontains a sulfur component--if sulfur is contained in the petroleumfeedstock--is deposited on the SO_(X) additive particles as well as onthe FCC catalyst particles.

Both kinds of particles, and hence the coke and sulfur deposited onthem, are carried from the FCC unit's reactor to its catalystregenerator. Here, the coke, and whatever sulfur that is contained inthat coke, is "burned off" both kinds of catalyst particles. The sulfurcomponent of such coke/sulfur deposits forms sulfur oxide gases (e.g.,sulfur dioxide and sulfur trioxide which are often collectively referredto as "SO_(X) " gases). Unless captured, these SO_(X) gases would beemitted to the atmosphere along with other flue gases given off by thecatalyst regenerator (e.g., carbon monoxide, carbon dioxide, nitrousoxides, etc.).

In other kinds of industrial facilities (e.g., coal-fired power plantsand certain chemical manufacturing plants), SO_(X) additives are usuallyemployed in the form of larger particles such as pellets that are notcirculated throughout the facility in the form of microspheroidalparticles, but rather are used in so-called "fluid bed" or "fixed bed"catalyst systems. In such systems, these catalyst pellets perform theirSO_(X) additive functions in a more localized region--as opposed tobeing circulated throughout the entire unit. These fixed bed and fluidbed systems are usually provided with so-called "swing reactors" whichprovide more than one fluid bed or fixed bed so that at least one bedcan be used to capture SO_(X) while at least one other bed is beingregenerated. Be these swing reactor configurations as they may, they tooproduce sulfur-containing flue gases. Thus, even though the sulfurcontained in the fossil fuels used to power electrical power plantsand/or provide process heat for chemical manufacturing facilities isconverted into SO_(x) gases in a manner somewhat different from that ofFCC units, the end result is the same;

unless captured, their SO_(X) emissions can and do enter, and pollute,the atmosphere.

Many materials have been used to prevent, or at least reduce, SO_(X)emissions from all such industrial facilities. The SO_(X) absorbingcomponent of these additives is normally a metal oxide of one kind oranother. Generally speaking, these metal oxides carry out their SO_(X)capturing function by forming metal sulfates when they are exposed toSO_(X) -containing gases, especially under high temperature conditions.A more complete identity of these metal oxides will be provided in laterportions of this patent disclosure.

Regardless of their identity, however, regeneration of "sulfated" SO_(X)additive particles usually involves converting them from their"contaminated" metal sulfate forms back to their "uncontaminated" metaloxide forms. For example, in the case of a FCC unit, the metal sulfateforms of the SO_(X) additive (that are produced in the catalystregenerator unit) are circulated, in admixture with regeneratedhydrocarbon cracking catalyst, from the catalyst regenerator unit backto the FCC unit's hydrocarbon cracking reactor zone. Here, the petroleumfeedstock is cracked and the sulfur components of the SO_(X) additiveparticles are converted to hydrogen sulfide gas by the hydrocarbon/hydrogen rich atmosphere existing in such reactor zones. As aconsequence of this, the metal sulfate component of a SO_(x) additive isreduced to its metal oxide form and, thus, is made ready for subsequentreuse in the catalyst regenerator. The hydrogen sulfide gas produced inthe FCC reactor unit is eventually captured and ultimately reduced toelemental sulfur in ways well known to the chemical engineering arts.

In the case of fluid bed or fixed bed catalyst systems such as thoseused to control SO_(X) emissions from power plants, the SO_(X) additiveis usually regenerated by passing a hydrocarbon-containing gas through aSO_(x) additive bed during a swing reactor regeneration cycle. Thisoperation also serves to convert those metal sulfates contained in theused, SO_(x) additive pellets back to their metal oxide forms. Methane,propane, and butane gases, as well as hydrogen gas itself, are used tocarry out the regeneration of such SO_(X) additives in these fixed bedor fluid bed systems.

Regardless of the exact nature of the industrial process being carriedout, and regardless of the physical size of the SO_(X) additivematerials being used, and regardless of the method used to regeneratesuch materials, any given SO_(X) additive system must perform at leastthree basic functions with respect to the sulfur oxide gases they seekto capture. First, these SO_(X) additive systems must oxidize SO₂ to SO₃; second, they must absorb the SO₃ once it is formed; and third, theymust be able to "give up" the captured SO₃ in order to be regenerated.The need to convert SO₂ to SO₃ follows from the fact that very fewmaterials are capable of both absorbing SO₂ gas and withstanding thehigh temperature conditions where the So₂ is created.

There are, however, many materials (e.g., various metal oxides) that areboth capable of absorbing SO₃ and withstanding the high temperatureenvironments where it is formed.

In most cases, these metal oxides are bivalent and/or trivalent metaloxides. For example, magnesia and/or alumina have been widely employedas SO_(X) additives in many different kinds of hydrocarbon catalyticcracking systems. By way of example, U.S. Pat. Nos. 4,423,019;3,835,031; 4,381,991; 4,497,902; 4,405,443 and 4,369,130 teach SO_(x)catalytic and/or absorbent activities for various metal oxides.

The prior art also has long recognized that certain metals (e.g.,cerium, vanadium, etc.) and their oxides (e.g., ceria, vanadia) can beemployed in SO_(X) additive systems in order to improve their ability tooxidize SO₂ to SO₃. Indeed, it might even be said that, to a very largedegree, the prior art with respect to using metal oxide materials asSO_(X) oxidants and/or absorbents has, for the past several decades,largely concerned itself with finding better ways of associating variouscatalytically active metals (e.g., cerium, vanadium, etc.) with allmanner of metal oxide materials in order to enhance the resultingmaterial's SO_(X) catalyzing and/or absorbing capabilities.

Some metal oxides also are known to improve the "release" of the sulfurcomponent of "used" SO_(X) additives when it comes time for them to bereduced back to their metal oxide forms. For example, U.S. Pat. No.4,589,978 ("the '978 patent") teaches SO_(X) transfer catalysts basedupon the use of rare earth metals such as cerium and lanthanum. The '978patent also teaches use of alumina to absorb SO₃ by forming aluminumsulfate in circumstances wherein the alumina is employed in the form ofa separate and distinct particle species that is used in admixture withother particles that contain a SO₂ →SO₃ oxidant. To these ends, the '978patent states: "The SO_(X) transfer catalyst of the present inventionpreferably includes a metal oxide such as alumina to absorb SO₃ assulfate. The alumina may be circulated as a separate particle or used asa support for the rare earth component. Preferably, the alumina is anactive form with high surface area, which includes synthetic alumina ingamma, theta, etc. forms as well as natural aluminas."

We would specifically note here that the '978 patent at least, inprinciple, recognizes that its alumina SO₃ -absorbing component "may becirculated as a separate particle . . . ". We also would note that theabove-quoted phrase goes on to say that its alumina may be "used as asupport for the rare earth component." Later, the '978 patent goes on tosay that its ". . . alumina and rare earth components can be furthersupported on an inert support or matrix which does not react with SO₂ orSO₃ to form sulfate. The supports for the alumina and rare earthoxidation component may be selected from silica, silica/alumina,zeolites, kieselguhr, celite or alumina." We have quoted these passagesfrom the '978 patent because, in some ways, the teachings of this patentreference define certain "points of departure" that help to establishand define the borders of the novel aspects of the invention describedin this patent disclosure. Therefore, the teachings of the '978 patentare incorporated herein in their entirety.

SUMMARY OF THE INVENTION

The present invention is based upon three conjunctive findings. Thefirst finding carries applicants' invention beyond the general teachingsof the '978 patent concerning the possible use of SO₂ oxidant catalystparticles that might be separate and distinct from the system's SO₃absorbent particles. The hereindescribed SO_(X) additive systems andprocesses--employing at least one SO₂ →SO₃ oxidation catalyst componentin a first particle species and at least one SO₃ absorbent component ina second particle species--require that at least one SO₂ →SO₃ oxidationcatalyst component of applicant's overall SO_(x) additive system bephysically separate and distinct from the SO₃ absorbent component ofsaid SO_(x) additive system. In other words, applicant's SO_(x) additivesystems will have at least two distinct particles species wherein thefirst particle species primarily carries out a SO₂ →SO₃ oxidationfunction. The second particle species will primarily carry out a SO₃absorption function. The second particle species may, however, alsocarry out a SO₂ →SO₃ oxidation function as well.

Applicants' second finding revolves around the fact that certainingredients are particularly effective in formulating each of thedifferent particle species of applicant's overall SO_(X) additivesystem. Applicant's third finding is that some of the ingredients taughtin the '978 patent (and, indeed, that are taught throughout the priorart regarding SO_(x) additives) should be used rather sparingly--orbetter yet, not used at all--in applicants' multi-particle, SO_(x)additive systems. More will be said about these limitations and/orprohibitions in subsequent parts of this patent disclosure.

The hereinafter described invention also is based upon applicants'recognition that use of most prior art SO_(x) additive systems dependsheavily upon some rather crude empiricisms that, for reasons that alsowill be hereinafter more fully described, result in a great deal ofwaste of the ingredients used to make SO_(X) additives. Morespecifically, applicants have recognized that there are situations inthe use of SO_(X) additives where an industrial plant operator may wantrelatively less SO₂ →SO₃ oxidation capacity, and relatively more SO₃absorption capacity in a given SO_(X) additive system. There also aresituations where just the opposite is true.

By way of example of the first situation, a FCC operator may want arelatively large amount of SO₂ oxidation capacity in those cases wherethe bulk, hydrocarbon cracking catalyst being employed in the FCC unitat a given time is itself also capable of absorbing some of the SO₃formed in the catalyst regenerator unit. This ability follows from thefact that some FCC, hydrocarbon cracking catalysts are made with matrixmaterials (e.g., alumina and magnesia) that will absorb SO₃ as well asserve as matrix-forming materials for the hydrocarbon cracking catalystparticles that are embedded in the overall catalyst particle (e.g.,magnesia and alumina are often employed in the matrix-forming materialsused to bind various zeolite particles that are employed in many bulk,hydrocarbon cracking catalyst). Many other hydrocarbon crackingcatalysts, however, are made with matrix-forming materials that havelittle or no SO₃ absorbing ability.

Thus, in those cases where a given hydrocarbon cracking catalyst beingemployed in an FCC unit also has the added ability to absorb SO₃, theoverall SO_(X) additive system (wherein the term "system" should betaken to imply the presence of at least one SO₂ →SO₃ oxidation catalystcomponent, and at least one SO₃ absorbent component) should containenough sulfur oxidant to produce all the SO₃ that the total catalystsystem can absorb--taking into consideration the fact that a given FCChydrocarbon cracking catalyst may itself have the ability to absorb someof the SO₃ produced by the sulfur oxidant catalyst. In other words, inthose cases where a given FCC hydrocarbon-cracking catalyst also has theability to absorb SO₃, the FCC operator would prefer a mixture of SO₂oxidant and SO₃ absorbent that is relatively "rich" in the oxidantcomponent of the SO_(X) additive system.

On the other hand, in those cases where the hydrocarbon crackingcatalyst matrix has little or no SO₃ absorption capability, little or no"extra" SO₂ →SO₃ oxidant is needed beyond what is needed to produce theamount of SO₃ that actually can be absorbed by the SO₃ -capturingcomponent of the SO_(X) additive system. The operator's adjustmentproblem (indeed, his dilemma) follows from the fact that,notwithstanding the teachings of the '978 patent concerning thepossibility of using separate SO₃ absorbents, commercially availableSO_(X) additive systems are formulated in the form of a single particlespecies that contains both the SO₂ →SO₃ oxidant component (e.g., ceria,vanadia, etc.) and the SO₃ absorbent component (e.g., alumina, magnesia,etc.). Thus, when using such single particle SO_(X) additive systems, ifthe operator wants to add more oxidant, more absorbent is inherentlyadded. Consequently, in certain operations, addition of more SO₂ →SO₃catalyst will "waste" some of the SO₃ absorbent capacity of any SO_(X)additive that is introduced into the unit in the form of a singleparticle species.

Conversely, a need for relatively more SO₃ absorbent occurs when aSO_(X) additive is provided to a FCC unit that is being used in a modeof operation known as "partial burn." In this mode of operation, thecombustion air admitted to the FCC regenerator unit is purposely limitedso that there only will be a small excess of oxygen in the flue gas.This is done in order to limit undesired combustion of carbon monoxideto carbon dioxide. Thus, in this mode of operation, a FCC operator wouldprefer to have a relatively larger amount of absorbent and a relativelysmaller amount of oxidant. That is to say that the operator would preferto limit the amount of oxidant so that there is only enough of it toconvert sulfur dioxide to sulfur trioxide without there also beingenough to convert carbon monoxide to carbon dioxide. In this case, ifthe operator wants to add more absorbent, more oxidant is inherentlyadded where only a single particle species SO_(X) additive is available.Consequently, the "extra" oxidant component of a single particle SO_(X)additive will be wasted during this mode of operation--indeed, it willbe detrimental to it.

FCC operators also experience many "upset" conditions. Not the least ofthese are those caused by changes in the sulfur concentration infeedstocks. Such upsets often require that changes be quickly made ineither the concentration of the SO_(x) oxidant or the concentration ofthe absorbent. Consequently, most operators usually respond to suchupset conditions by immediately addressing the most pressing upsetcondition existing at the moment--regardless of waste of "unused"component oxidant, or waste of "unused" absorbent, that may be containedin a single particle SO_(x) additive system. Indeed, many such upsetscould be prevented if the plant operator had a multi-particle SO_(x)additive system at his command.

Applicants also have found that there are even more subtleconsiderations that can be addressed through independent addition ofeither the SO₂ →SO₃ oxidant catalyst, or the SO₃ absorbent. They revolvearound the "aging rate" of the oxidant component of an SO_(X) additivesystem relative to the usually different "aging rate" of its SO₃absorbent component. In actual industrial operations such as petroleumcracking, there is no presently known way of accurately predicting theuseful life of either the oxidation component, or the absorbentcomponent, of a given SO_(X) additive in a given FCC unit. That is tosay that depending on such factors as the nature of the feedstock beingprocessed, the product being produced and the mechanical features of agiven refinery, the SO₂ oxidant may "die" before the SO₃ absorbent, orvice-versa. Thus, in a SO_(X) is additive system comprised of only asingle particle species that contains both a SO_(X) oxidation componentand a SO_(X) absorbent component, there is usually a subtle waste offunctional capacity of one or the other of these two components becausethey rarely "die" at the same time. In other words, in a single particlespecies SO_(X) additive system, one of these two components usually dieswhile the other component still has some remaining "useful life". Ineffect, the hereindescribed compositions and processes provide a methodfor "using up" any remaining useful life in either of these twocomponents. Therefore, a major advantage of applicant's multi-particleSO_(X) additive systems--relative to single particle SO_(X) additivesystems--is their ability to maximize usage of each of the two mainactive components of an SO_(X) additive system (i.e., the SO₂ →SO₃oxidant component and the SO₃ absorbent component), regardless of howthey age relative to each other in any given industrial facility.

Thus, overall, use of the hereindescribed compositions and processesprovide the FCC operator with many operating advantages that are notattainable through use of single particle species, SO_(X) additivesystems. These advantages include greater flexibility in: (1)compensating for cracking catalyst that do--or do not--have SO₃absorption capabilities, (2) controlling partial burn operations, (3)preventing, and more effectively and efficiently controlling upsets inthe operation of a given industrial facility and (4) more completelyutilizing all of the active ingredients in a given SO_(X) additivesystem under ever-varying operating conditions.

General Nature of Applicant's SO_(x) Additive Systems

The first component of applicants' overall SO_(X) additive system is aSO₂ →SO₃ oxidation catalyst component. This component can be made andused in the form of microspheroidal particles, pellets, lumps, etc.depending upon its intended end use. The second component is a SO₃absorbent component. It too can be made and used in the form ofparticles, pellets, etc. Thus, for the purposes of this patentdisclosure, the term "particle(s)" should be taken to include thosepellets used in fixed bed and moving systems--as well as those smaller,microspheroidal particles used in FCC operations. In order to practicethis particular invention however, at least one SO₃ absorbent componentparticle species must be used in the form of physically separate anddistinct particles, pellets, etc. from at least one SO₂ →SO₃ oxidationcatalyst particle species.

The relative proportions of the SO₂ →SO₃ oxidation catalyst component tothe SO₃ absorbent component can vary considerably in the practice ofthis invention. For example a SO₂ →SO₃ oxidation catalyst component(which may comprise one or more species of SO₂ →SO₃ oxidationparticle(s)) can comprise from about 10 to about 90 weight percent ofapplicants' overall SO_(x) additive system. Similarly, the SO₃ absorbentcomponent (which may, likewise, comprise one or more species of SO₃absorbent particle(s)) can constitute from about 10 to about 90 weightpercent of the additive system. These two components can be separatelyintroduced into a given industrial facility, or they can be premixed andintroduced into such a facility as a mixture.

It also should be noted that applicants' SO₂ →SO₃ catalyst componentsmay inherently have some SO₃ absorbent capability and that applicants'SO₃ absorbent components may inherently have some SO₂ →SO₃ oxidationcatalyst ability. Nonetheless, practice of this invention requires thatat least one of applicants' particle species primarily carry out a SO₂oxidation function while and at least one other, physically separate anddistinct particle species, carries out the SO₃ absorption function. ThisSO₃ absorption function may be the only duty of the SO₃ absorbentcomponent. In some embodiments of this invention, however, the SO₃absorbent component also may be provided with a SO₂ →SO₃ oxidationcatalyst so that the resulting material is capable of carrying a SO₂oxidation function as well as a SO₃ absorption function.

The SO₂ →SO₃ oxidation component of the hereindescribed SO_(x) additivesystems may itself be comprised of two or more separate and distinctparticle species. For example a first SO₂ →SO₃ oxidation catalystcomponent could employ ceria as its SO₂ →SO₃ oxidation ingredient whilea second SO₂ →SO₃ oxidant catalyst component employs vanadia as its SO₂→SO₃ oxidant ingredient. The same is also true of applicants' SO₃absorbent component. For example, a first SO₃ absorbent component couldemploy a calcium oxide or calcium aluminate SO₃ absorbent while a secondSO₃ absorbent component employs a magnesium oxide SO₃ absorbentcomponent. And, as was noted in the preceding paragraph, at least oneSO₃ absorbent catalyst species can be provided with an SO₂ →SO₃oxidation catalyst and thereby simultaneously serve as a "second"oxidation catalyst component as well as a SO₃ absorbent component. Insuch cases, the SO₃ absorbent will serve to pick up SO₃ produced by theseparate and distinct SO₂ →SO₃ oxidation catalyst particle as well asthe SO₃ produced by the SO₂ oxidant in the SO₃ absorbent particleitself.

Such SO₂ →SO₃ oxidation catalysts and such SO₃ absorbents can be used inassociation with the same kind of, or with different, support materials.Applicants also have found that their oxidation catalyst component(s)and their SO_(x) absorbent component(s) are preferably used in admixturewith each other--as opposed to being used sequentially--that is to sayby locating a zone where the SO₃ absorption takes place "down stream"from a different zone where the SO₂ →SO₃ oxidation takes place. And as afinal note on the general use of the hereindescribed SO_(X) additivesystems, applicants would note that their SO_(x) additive systems can beused with a very wide variety of hydrocarbon cracking catalysts. At thevery least, such hydrocarbon cracking catalysts would include any ofthose natural or synthetic crystalline aluminosilicate zeolites (e.g.,faujasite zeolites of the X and Y type) commonly used for such purposes,as well as various amorphous metal oxides, (e.g., amorphous alumina)having hydrocarbon cracking activities. Those skilled in this art willappreciate that a hydrocarbon cracking catalyst component willconstitute a major portion of those hydrocarbon cracking catalyst/SO_(x)additive mixtures used in industrial facilities such as FCC units. Forthe most part, applicants' SO_(x) additives will comprise only fromabout 0.5 to about 10.0 weight percent of such hydrocarbon crackingcatalysts/SO_(x) additive mixtures.

SO₂ →SO₃ Oxidation Catalyst Components

Applicants' SO₂ →SO₃ oxidation catalyst component is comprised of atleast two general kinds of ingredients. The first of these two generalkinds of ingredients is a sulfur dioxide oxidation catalyst ingredientthat is inherently capable of oxidizing SO₂ to SO₃ in an environmentwhere the SO₂ is created. Most preferably, this SO₂ →SO₃ oxidationcatalyst ingredient will comprise: (1) a metal selected from the groupconsisting of those metals having an atomic number of at least 20, ametal from Groups 1b and 2b of the Periodic Table, a metal from GroupsIII to VIII of the Periodic Table, and/or a rare earth metal of thePeriodic Table. Of these metals, cerium, vanadium, platinum, palladium,rhodium, iridium, molybdenum, tungsten, copper, chromium nickel,manganese, cobalt, iron, ytterbium and uranium are preferred. And, ofthese, cerium and vanadium are the most preferred--and especially whenthey are used in conjunction with each other--but not necessarily in thesame particle species.

These metals may be employed in their "free" or uncompounded metallicforms (e.g., metallic platinum) as well as in chemically compoundedforms (e.g., in the form of their oxides) in applicants' end products.Thus the term "metal", for the purposes of this patent disclosure shouldbe taken to mean chemically compounded metals as well as uncompounded,elemental metals. It also should be noted that when these metals arefirst introduced into the "wet" reaction systems used in making thehereindescribed SO_(x) additives, they may be in various salt forms,e.g., their oxides, nitrates, acetates, and carbonates--indeed, in mostcases, the salt forms of these metals are generally preferred over theirelemental, metallic forms as starting ingredients for applicants' SO₂→SO₃ catalysts. For example, the preferred forms of cerium inapplicants' wet reaction systems are ceria, cerium acetate, ceriumnitrate and cerium carbonate. Moreover, some metal salt forms such as aceria component of the wet reaction systems used to create thehereindescribed SO_(x) additives may, in turn, be previously prepared bydecomposing their various other salt forms such as cerium acetate,cerium nitrate, or cerium carbonate. Similarly, the more preferred formsof vanadium in applicants' SO_(X) catalyst components will includevanadium oxide, and/or the decomposition products of various vanadiumsalts such as those of ammonium vanadate or vanadium oxalate.

Next, it should be noted that the calcination step of thehereindescribed processes will serve to convert any non-oxideingredients (e.g., cerium acetate, cerium nitrate, cerium carbonate,etc.) used in the wet reaction systems into their oxide forms (e.g.,cerium acetate, nitrate, carbonate, etc. will be converted to ceriumoxide). That is to say that a component of a SO_(x) catalyst orabsorbent of this patent disclosure (e.g., a binder component) can bemade from a non-oxide form of the metal that is used in the "wet"reaction system, but is subsequently converted to an oxidation of thatmetal by the calcination step of applicants' process. It also should benoted that these SO₂ →SO₃ oxidizing metals may be associated with thebinder by placing them in the reaction mixtures along with the binderingredients; or they may be made by impregnating solutions containingions of these metals into a dried form of applicants' SO₂ →SO₃ oxidationcatalyst forming materials or dried forms of their SO₃ absorbentmaterials.

Supports for the Oxidants

The second general kind of ingredient in applicants' SO₂ →SO₃ oxidationcatalyst component is a binder (or support) material for the SO₂ →SO₃oxidation catalyst ingredient. For the purposes of this patentdisclosure the terms "binder" and "support" should be regarded as beingequivalent. Such binder or support materials preferably are made frommetal oxide ingredients selected from the group consisting of calciumaluminate, aluminum silicate, magnesium aluminate, aluminum titanate,zinc titanate, aluminum zirconate, magnesia, alumina (Al₂ O₃), aluminumhydroxide compounds, aluminum-containing metal oxide compounds (otherthan alumina (Al₂ O₃) or aluminum hydroxide compounds), zirconia,titania, silica, bastnaesite, various clays (and especially kaolinclay), and/or clay/phosphate materials such as those taught in U.S. Pat.Nos. 5,190,902 and 5,288,739 (hence, the teachings of these two patentsare hereby incorporated by reference into this patent disclosure).

Limitations Re: Supports for the Oxidants

The second and third aspects of this invention revolve round applicants'further findings that in order to effectively use the hereindescribedmulti-particle SO_(X) additive systems, the oxidant support materialsand SO₃ absorbent materials of applicants' SO_(X) additive systems mustbe made with large proportions of certain materials, and not made withcertain other less undesired materials, or, at the very least, be madewith relatively little of the undesired materials. These limitationsand/or prohibitions in formulating applicants' multi-particle, SO_(X)additive systems form a part of the overall novelty of this inventionbecause the hereinafter identified "low concentration", or "prohibited",ingredients are presently used, in large proportions, in formulating awide variety of "single particle species," SO_(X) additives.

For example, applicants have found that the amount of alumina (Al₂ O₃)used in their support materials for their oxidation catalyst ingredientsshould not constitute any more than about 10 weight percent of theoverall oxidation catalyst component. More preferably, applicants'oxidation catalyst components will contain no alumina (Al₂ O₃)whatsoever. This stands in sharp contrast to the fact that many singleparticle species, SO_(x) additives are often comprised of from 50 to 95%alumina (Al₂ O₃). Similarly, applicants have found that the amount ofaluminum hydroxide compounds (as the term "aluminum hydroxide" isdefined in the next paragraph) should not constitute any more than about10 weight percent of the oxidation catalyst component of the SO_(x)additive systems of this patent disclosure. And here again, it is evenmore preferred that no aluminum hydroxide compound whatsoever be used inapplicants' oxidation catalyst components. This too, stands in sharpcontradistinction to the fact that many aluminum hydroxide compounds arelikewise used in very high concentrations in many single particle,SO_(x) additives.

Be that as it may, for the purposes of this patent disclosure, the term"aluminum hydroxide compound(s)" should be taken to mean aluminumhydroxide in any of its many phase forms. For example, an aluminumhydroxide classification "tree diagram" such as that found on page 9,ACS Monograph 184, Industrial Alumina Chemicals, Misra, Chanakya, 1986,which is incorporated by reference herein, shows that the term "aluminumhydroxide" can have a rather broad meaning that includes many differentphase forms of that compound. For example, this reference points outthat there are "crystalline" forms of aluminum hydroxide that include afirst group of crystalline trihydroxides Al(OH)₃ whose members aregibbsite, bayerite, and nordstrandite. This classification diagram alsoshows a second group ("Oxide-Hydroxides AlOOH") of crystalline, aluminumhydroxides comprised of boehmite and diaspore. Another separate anddistinct group of aluminum hydroxides in this classification diagram isfound in another branch of the tree diagram under the heading"gelatinous"--which is distinguished from the "crystalline" forms ofaluminum hydroxide noted above. The gelatinous group is comprised ofpseudoboehmite and X-Ray indifferent aluminum hydroxide (which is alsooften referred to as "amorphous alumina"). In any case, all of thesematerials should be considered as "aluminum hydroxide compound(s)" forthe purposes of this patent disclosure and their concentrations in theoxidation catalyst components of this patent disclosure should not bemore than about 10% by weight of said oxidation catalyst component.Better yet, these materials should not be used at all in applicants' SO₂→SO₃ oxidation catalyst components. Again, these prohibitions andlimitations are quite unexpected since alumina (Al₂ O₃) and many ofthese alumina hydroxide compounds have been widely used, in very largeproportions, in many prior art, single particle species, SO_(x)additives.

By way of contrast, aluminum-containing compounds (that are not aluminaor aluminum hydroxides) that are suitable for use in applicants'oxidation catalyst components, in proportions larger than 10 weightpercent, would include compounds wherein aluminum is chemically reactedwith elements, or groups of elements, other than the oxygen of alumina(Al₂ O₃) or the (OH)⁻ groups found in the aluminum hydroxide compoundsnoted in the preceding paragraph. Examples of such aluminum-containingcompounds (that are not alumina or aluminum hydroxides) would includealuminum silicate, aluminum titanate, aluminum zirconate and magnesiumaluminate. Such aluminum-containing compounds may constitute from about5 to about 99 percent of applicants' overall oxidation catalystcomponent. The other materials suitable for use as support materials inapplicants' oxidation catalyst components (e.g., magnesia, zirconia,titania, silica, bastnaesite, kaolin clay and/or clay-phosphatematerials, etc.) may likewise constitute from about 5 to about 99 weightpercent of the oxidation catalyst components of this patent disclosure.As a final note with respect to those other materials suitable for useas support materials for the oxidation catalyst component of applicants'SO_(x) additive system, it is preferred that when the oxidation catalystingredient is a platinum group metal, then the support material shouldcontain relatively little or no silica (e.g., no more than about 10weight percent of the oxidation catalyst component).

SO₃ Absorbent Components

Although one material (e.g., a hydrotalcite or magnesia/alumina solidsolution such as that prepared according to Example 1 herein) may serveas both an SO₃ absorbent and as its own binder material in the practiceof this invention, applicants' SO₃ absorbent component will, however,more preferably comprise at least one sulfur trioxide (SO₃) absorbentingredient and at least one, chemically different, support material forthat SO₃ absorbent ingredient. For example, a SO_(x) additive may becomprised of a hydrotalcite SO₃ absorbent supported by a calciumaluminate binder.

Regardless of the identity of the support ingredient, all such SO₃absorbent ingredient(s) will be selected primarily for their ability toboth "pick up" and "give up" sulfur trioxide. Metal oxides selected fromthe group consisting of hydrotalcite, hydrotalcite-like compounds,magnesia, alumina, calcium aluminate and calcium oxide are particularlyeffective as SO₃ absorbents in the practice of this invention. Otheruseful ingredients for creating SO₃ absorbents will include magnesiumacetate, magnesium nitrate, magnesium hydroxide, magnesium carbonate,magnesium formate, magnesium chloride, magnesium aluminate, hydrousmagnesium silicate, magnesium calcium silicate, calcium silicate, aswell as other magnesium-containing compounds.

Of these materials, hydrotalcite and various hydrotalcite-like materialsare particularly preferred. Those skilled in this art will appreciatethat the material commonly referred to as "hydrotalcite" has a magnesiumaluminate hydroxy carbonate structure with the classical formula Mg₆ Al₂(OH)₁₆ CO₃ -4H₂ O. This material is described in ICDD (InternationalCenter for Diffraction Data) Card Number 22-0700 as "magnesium aluminumcarbonate hydroxide hydrate/hydrotalcite." The chief differentiatingcharacteristics of materials possessing this hydrotalcite structureinclude the unique x-ray diffraction pattern depicted in ICDD CardNumber 22-0700 as well as the material's ability to experience anendothermic reaction at about 300-450° C. This reaction corresponds tothe loss of both the OH water (water of hydration) and CO₂. Thisstructural change is however reversible, meaning that once heated above450° C., further reaction with water will cause this hydrotalcitestructure to reappear, as can be verified by subsequent x-raydiffraction tests, and the resulting material will again show an abilityto undergo its characteristic endothermic reaction.

Next, it should be noted that, strictly speaking, the above statementsapply to hydrotalcite having the above-noted classical formula. It isalso the case however that changes in the processing conditions(composition, temperature, pressure, humidity, etc.) used to makecatalytic materials containing hydrotalcite also can result inproduction of materials having non-stoichiometric hydrotalcitestructures and which possess XRD patterns which differ from those givenin ICDD Card Number 22-0700. Such hydrotalcite-like materials willinclude (but not be limited to) other magnesium aluminumhydroxide-containing compounds, e.g., magnesium aluminum hydroxidehydrate (ICDD Card No. 35-0965), magnesium aluminum hydrate (ICDD CardNo. 35-1275) and magnesium aluminum hydroxide hydrate (ICDD Card No.35-0964). Although an exact match may not always be made by XRD, suchstructures can be further surmised empirically by running various tests(e.g., differential scanning calorimetry (DSC) and thermogravimetricanalysis (TGA)) on a given sample. This is based upon the fact that thebonding of certain components (e.g., carbonate and/or OH water) to such"hydrotalcite-like" structures will have a certain energy related to itwith respect to breaking the bonds and a subsequent absorption of heat.Also accompanying such reactions is a loss in mass of the sample. TGAcan measure such losses while DSC procedures can measure both the typeof reaction as well as the heat absorbed or evolved.

Next it should be noted that since the ability of such hydrotalcite-likematerials to absorb SO_(x) is not particularly dependent on theircompositions strictly adhering to the previously noted classical formulafor hydrotalcite, many other non-stoichiometric compositions ofhydrotalcite and hydrotalcite-like composition also can be used in thepractice of this invention. Preferably, all of these compositions shouldcontain OH water and/or carbonate in their molecular structures.Consequently, compositions possessing such hydrotalcite-like structuresmay be employed for the practice of this invention. Such"hydrotalcite-like" materials would include, but by no means be limitedto, Mannaseite and Indigirite. Those skilled in this art also willappreciate the literature also frequently refers to such materials"anionic clay minerals," although they may be synthetically produced. Inany case, those anionic clay minerals that contain magnesium andaluminum, are particularly preferred for the practice of this invention.Thus, for the purposes of this patent disclosure, applicants' use of theterm "hydrotalcite" should be taken to include not only hydrotalciteforms having the above-noted classical formula, but alsohydrotalcite-like structures or anionic clay structures such as those ofManasseite and Indigirite.

Next it should be again noted that applicants' SO₃ absorbent componentsalso may be provided with their own SO₂ →SO₃ oxidation catalystingredient(s). By way of example only, one or more of the particlespecies that make up applicants' SO₃ absorbent component may--as anoption, and not a requirement--be provided with SO₂ →SO₃ catalystsselected from the group consisting of cerium, vanadium, platinum,palladium, rhodium, iridium, molybdenum, tungsten, copper, chromium,nickel, manganese, cobalt, iron, ytterbium, and uranium. Of thesepossible SO₂ →SO₃ oxidation catalysts, vanadia has proven to be aparticularly effective SO₂ oxidation catalyst in the context of adding aSO₂ oxidant to applicants' SO₃ absorbent component(s). It also should benoted in passing that when vanadia is used in a SO₃ absorbent component,it is highly preferred that the SO₃ absorbent also have a magnesiacomponent as well. Applicants have found that the presence of magnesiain their SO₃ absorbent component(s) serves to prevent the "escape" ofthe vanadia from the SO₃ absorbent--and thereby preventing "vanadiapoisoning" of the hydrocarbon cracking catalyst with which these SO_(x)additives are used.

And, as was the case with applicants' SO₂ →SO₃ oxidation catalystcomponents, the SO₃ absorbent starting ingredient metal(s) can be intheir oxide, acetate, nitrate, chloride, carbonate, formate, etc. formswhen they are first introduced into the wet reaction systems employed toformulate these SO_(X) absorbent components. Again, the metal salt formsthat are most suitable for creating applicants' SO₃ absorbent componentsare preferably selected from the group consisting of calcium aluminate,hydrotalcite, hydrotalcite-like compounds, calcium oxide, aluminumoxide, magnesium oxide (and especially its periclase phase), magnesiumacetate, magnesium nitrate, magnesium chloride, magnesium hydroxide,magnesium carbonate, magnesium formate, magnesium aluminates, hydrousmagnesium silicates (e.g., talc), magnesium calcium silicates (e.g.,dolomite), and calcium silicate (e.g., wollastonite). Here again,however, applicants' calcination step will usually convert any of thenon-oxide metal forms of these salts to their metal oxide forms.

Supports for SO₃ Absorbent Ingredients

Applicants' SO₃ absorbent ingredients are preferably used in conjunctionwith a support material selected from the group consisting of calciumaluminate, aluminum nitrohydrate, aluminum chlorohydrate, magnesia,silica (SiO₂), silicon-containing compounds (other than silica), alumina(Al₂ O₃), titania, zirconia, various clays and/or clay-phosphatematerials (and especially those whose phosphate source is phosphoricacid or dibasic ammonium phosphate e.g., those produced by the methodstaught in U.S. Pat. Nos. 5,190,902 and 5,288,739). Again, the presenceof magnesia in the SO₃ absorbent is highly preferred when said absorbentalso has a vanadia component in the same particle.

Next, it should be noted that, for the purposes of this patentdisclosure, the term "silica" should be taken to mean silicon dioxide(SiO₂) in any of its various phase forms. By way of contrast with theterm "silica", applicants intend that the term "silicon-containingcompounds (other than silica)" should be taken to mean a compoundwherein a three dimensional network of cations and oxygen is formedwhich consists of silica tetrahedra and at least one other non-siliconcontaining oxide component. Examples of such silicon-containingcompounds falling under applicants' use of this term would includecalcium silicate, magnesium silicate, and a variety of aluminumsilicates and, hence, a wide variety of clay materials that containsilicates (e.g., kaolinite, serpentine, pyrophyllite, talc, smectitemontmorillonite, vermiculite, illite, mica, brittle mica, chlorite andpalygorskite).

Additional Findings Re: Supports for SO₃ Absorbent Ingredients

Applicants have found that in the case of silica (SiO₂), no more thanabout 10 weight percent of it should be used in any one particle speciesof applicants' SO₃ absorbent component. It is even more preferred thatno silica (SiO₂) whatsoever be so employed. Applicants have also foundthat when kaolin clay is used in their SO₃ absorbent components, it tooshould be used in somewhat limited proportions; applicants have foundfor example that, contrary to the fact that many single particle speciesSO_(x) additives have kaolin concentrations up to 90 percent, kaolinshould not constitute more than about 50 weight percent of any givenparticle species of applicants' SO₃ absorbent component--and it is evenmore preferred that no more about 30% of it be used in any given SO₃absorbent component particle.

Applicants also would note that, for the purposes of this patentdisclosure, the term "kaolin clay" should be taken to mean those clayscomposed of layers of silica and alumina sheets wherein the ratio ofsilica sheet to alumina sheet gives rise to a 1:1, dimorphic, ortwo-sheet, kaolin clay unit wherein the tips of the silica tetrahedraproject into an hydroxyl plane of the material's octahedral sheet andreplaces two thirds of the hydroxyl ions (bearing in mind that thealumina sheet in such clays has an upper and a lower plane bothconsisting of hydroxyl ions between which is a plane of Al³⁺ ions, thatare octahedrally coordinated to the hydroxyl groups). Be this definitionas it may, applicants' limitations regarding kaolin are also rathersurprising in that this type of clay has been widely used, in largeconcentrations, in many prior art, single particle, SO_(x) additives.This admonition-limitation regarding the use of kaolin clay inapplicants' SO₃ absorbent components also stands in sharp contrast tothe fact that kaolin can be used, in large proportions, in the SO₂ →SO₃oxidation catalyst component of applicants' SO_(x) additive systems.

Silicon can however be used in applicants' SO₃ absorbent components whenit is a part of various compounds other than silica and kaolin clay. Forexample, silicon employed in the form of magnesium silicate, aluminumsilicate, silicon titanate, and/or silicon zirconate may constitute upto about 40 weight percent of these support materials for the SO₃absorbent. Applicants also prefer that those atoms or groups of atomsthat are chemically combined with a silicon component of suchnon-kaolin, silicon containing compounds be present in excess of thestoichiometric amount required to react with the silicon. And as a finalnote on the subject of silicon-containing compounds (other than kaolin),applicants have generally found that use of aluminum silicate inapplicants' SO₃ absorbents produces especially good results.

Still Further Findings Re: SO₃ Absorbents

Applicants have noted that, contrary to single particle, SO_(X) additivesystems wherein only a rather limited choice of absorbents is available(e.g., the preferred SO_(x) absorbent materials in such single particleSO_(x) additives being magnesia and alumina), use of applicants'multi-particle SO_(x) additive systems considerably broadens applicants'choice of the SO₃ absorbents to include many other materials that wereheretofore unacceptable for use in single particle SO_(x) additives. Forexample, oxides of the metallic elements in Group 1a and 2a of thePeriodic Table have not been widely used in SO_(x) additives, but servenicely as SO_(x) absorbents in applicants' SO₃ absorbent components. Andthe same is also true of various oxides of certain rare earth metalse.g., cerium, lanthanum and praseodymium.

Applicants' comparative experimental work with respect to the relativemerits of single particle species SO_(x) additives versus themulti-particle SO_(x) additive systems of this patent disclosuregenerally showed that most prior art SO₃ absorbents can be divided intothree broad categories; those that are clearly unsuitable for use in FCCunits, those that absorb moderate amounts of SO₃ and are very stable insuch units, and those that absorb large amounts of SO₃, but are not verystable in such high temperature environments. Generally speaking,applicants found that in using single particle species SO_(x) additives(those having both SO2 oxidants and SO₃ absorbents in the sameparticle), emphasis is better placed upon materials with moderateabsorption capabilities and good stability. In applicants'multi-particle SO_(x) additive systems, however, it has been found thatthere are instances where materials with high SO₃ absorption and lowstability can be used to great advantage. For example, the results ofapplicants' studies regarding the relative absorption capabilities ofabsorbents made from various magnesium based materials are shown inTable I.

                  TABLE I                                                         ______________________________________                                        Magnesia Source Absorption, % Weight                                          ______________________________________                                        Dolomite        78                                                            MgO (first type)                                                                              110                                                           MgO (second type)                                                                             147                                                           MgO (third type)                                                                              151                                                           MgO (fourth type)                                                                             142                                                           Magnesium Acetate                                                                             74                                                            Magnesium Nitrate                                                                             11                                                            Magnesium Chloride                                                                            57                                                            Magnesium Hydroxide                                                                           79                                                            Magnesium Carbonate                                                                           144                                                           Magnesium Formate                                                                             105                                                           Hydrotalcite    --                                                            ______________________________________                                    

Some of the more absorbent magnesium-containing compounds performed verywell in applicants' SO_(x) absorbent components even though they do notperform particularly well in single particle SO_(x) additives.Applicants also have found that magnesia is particularly effective as aSO₃ absorbent ingredient in their SO_(x) absorbent components when themagnesia ingredient is obtained from thermal decomposition of certainmagnesia salts such as magnesium acetate, magnesium hydroxy acetate andmagnesium formate.

Additional Findings Re: Hardening Agents

One of the major disadvantages of using magnesia in applicants' SO₃absorbents is that particles that contain large percentages of it tendto be too "soft" for use in FCC units. To correct this deficiency withrespect to FCC applications (fluid-bed and fixed-bed systems are not assensitive to this concern), applicants found that use of certain wellknown hardening agents can be used to great advantage. In general,applicants have found that such hardening agents should be powderedmaterials that are insoluble in the reaction slurry used to formulatethe ingredients for the SO₃ absorbent component. Preferably, powderedmaterials will have particles whose average diameters of less than about2.5 microns. It is also preferred that these hardening agents not bechemically reactive with the other ingredients in the wet reactionslurry into which they are introduced. Another desired property of thesehardening agents is their own inherent ability to absorb SO₃. To theseends, applicants have found that some particularly useful hardeningagents for the practice of this invention are various forms of aluminumsilicates, magnesium aluminates, magnesium silicate and magnesiumcalcium silicate. Moreover, certain natural and synthetic clays, thatare active cracking catalysts, also can be used to advantage ashardening agents in applicants' SO₃ absorbent components since they toocontain active sites that are capable of absorbing SO₃. Examples ofclays useful in this regard would include halloysite, rectorite,hectorite, montmorillinite, synthetic montmorillinite, sepiolite,activated sepiolite and, with the previously noted proportion limitation(i.e., not more than 50 weight percent of the SO₃ absorbent component),kaolin.

Expressed in patent claim language, a particularly preferred embodimentof applicants' SO_(x) additive systems will comprise a SO₂ →SO₃oxidation catalyst component and an SO₃ absorbent component wherein: (1)the SO₂ →SO₃ oxidation catalyst is comprised of a metal selected fromthe group consisting of a metal having an atomic number of at least 20,a metal from Group 1B of the Periodic Table, a metal from Group 11B ofthe Periodic Table, a metal from Groups III to VIII of the PeriodicTable and a rare earth metal of the Periodic Table; and (b) a binderselected from the group of metal-containing compounds consisting ofcalcium aluminate, aluminum silicate, aluminum titanate, aluminumzirconate, zinc titanate, magnesia, alumina, aluminum-containing metaloxide compound, aluminum hydroxide, clay, zirconia, titania, silica,clay/phosphate material and bastnaesite and which, if employed at all,contains no more than about 10 weight percent aluminum hydroxide and nomore than about 10 weight percent alumina (Al₂ O₃); and (2) a SO₃absorbent component that is physically separate and distinct from theSO₂ →SO₃ oxidation catalyst component and comprises: (a) a metal oxideselected from the group consisting of magnesia, alumina, calciumaluminate, calcium oxide, hydrotalcite, hydrotalcite-like compounds and,as an optional ingredient, also contains a support material selectedfrom the group of metal oxides consisting of calcium aluminate,magnesia, alumina, silica, kaolin clay, titania clays, clay/phosphatematerial and zirconia and, which, if employed at all, contains no morethan about 10 weight percent silica and no more than about 50 weightpercent kaolin clay.

FURTHER DETAILED DESCRIPTIONS OF THE INVENTION Relative Proportions ofIngredients

The relative proportions of the various ingredients in applicants'initial, "wet" reaction compositions, will not equal the relativeproportions of those ingredients in the "dry", post-calcined, endproducts (e.g., in the microspheroidal particle or pellet forms in whichapplicants' products are ultimately made and then used). This followsfrom the fact that the liquid media and certain volatile ingredient(s)that are used to create applicants' initial, wet, reaction compositionswill be virtually completely driven off during applicants' subsequentspray drying and calcining steps. Indeed, many of these ingredients willundergo a change in their chemical identity as a result of applicants'calcination step. For example, magnesium acetate, carbonate, nitrate,etc. ingredients will each be converted to magnesium oxide as a resultof the calcination step of applicants' overall manufacturing process.Thus, TABLE II below gives the broad ranges and the preferredconcentration for the various ingredients that comprise applicants'post-calcined, "dry", end product SO_(x) additives. It also should beunderstood that many optional ingredients (other than optional hardeningagent ingredients that take the form of metal oxides) can be used toenhance the manufacturing of either the SO₂ →SO₃ oxidation catalystcomponent or the SO₃ absorbent component of applicants' SO_(x) additivesystems. These optional ingredients might include viscosity agents, gasevolution agents, etc. If they are used, they will usually constitutefrom about 1 to about 20 weight percent of a given "wet" reactionmixture. For the most part, however, most optional ingredients of theselatter kinds will be used to enhance the manufacturing process, but willbe completely driven off by applicant's spray drying and/or calcinationsteps. In any event, regarding the ingredients that are found inapplicants' post-calcined materials, the preferred concentrationsthereof are:

                  TABLE II                                                        ______________________________________                                        SO.sub.x Additive Systems                                                                     Wt. % Wt. %                                                                   Range Preferred                                               ______________________________________                                        SO.sub.2 → SO.sub.3 Oxidation                                          Catalyst Component                                                            SO.sub.2 → SO.sub.3 oxidant                                                              5 to 95 30                                                  Binder material   5 to 95 70                                                  Total             100%    100%                                                SO.sub.3 Absorbent                                                            Component                                                                     SO.sub.3 absorbent                                                                              5 to 100                                                                              60                                                  SO.sub.3 Support  5 to 100                                                                              30                                                  Optional Hardening                                                                               0 to 20                                                                               8                                                  Agent Ingredient(s)                                                           Optional SO.sub.2 → SO.sub.3                                                             0 to 5   2                                                  Oxidation Catalyst                                                            Ingredient                                                                    Total             100%    100%                                                ______________________________________                                    

Spray Drying Operations

When they are used, the spray drying processes used to createmicrospheroidal forms of applicants' SO₂ →SO₃ oxidant catalystcomponents and SO₃ absorbent components each can be carried out by wellknown techniques. Generally speaking, such spray drying should bedesigned to produce microspheroidal particles having a range of sizessuch that essentially all such particles will be retained by a StandardU.S. 200 mesh screen and essentially all such particles will be passedby a Standard U.S. 60 mesh screen.

Optional Drying Procedures

It should also be noted that in addition to a spray drying step that maybe used to produce microspheroidal forms of applicants' SO₂ →SO₃oxidation catalyst components and SO₃ absorbent components, thehereindescribed manufacturing processes may be further enhanced by useof separate and distinct drying steps. They normally will be carried outafter the drying which naturally results from the spray drying step (orfrom those other drying steps used to produce pellet forms ofapplicants' materials). Such additional drying will further serve toremove any remaining traces of the liquid medium used to create thereaction mixtures which may be still present in the interstices of theparticles and/or associated with such materials as water of hydration.Drying times for such distinct drying steps will normally take fromabout 0.2 hours to about 24 hours at temperatures which preferably rangefrom about 200° F. to about 500° F. (at atmospheric pressure), but inall cases, at temperatures greater than the boiling point of the liquidmedium employed (e.g., greater than 212° F. in the case of water) in the"wet" reaction compositions.

Calcining Procedures

After spray drying (in the case of microspheroidal particles) ordesiccation (in the case of pellet forms of applicants' materials)--itremains only to take the solid matrix of the anhydrous materials thusproduced and convert their various non-oxide ingredients to their oxideforms. This is preferably done by a calcination step. In effect, thecalcination step serves to drive off any volatile components and replacethem with oxygen and thereby produce a final product havingpredominantly the oxide forms of the ingredients. The calcination stepalso serves to drive off, as gaseous oxides, all but the "desirable"components of the resulting metal oxide materials. For example, thiscalcination step will drive off the liquid media and any acid, viscosityagent and/or gas evolution agents that may have been used in theoriginal wet, reaction mixture that existed before drying procedureswere implemented. Such calcination is readily accomplished by heatingthe products of the spray drying step--or of the optional desiccationstep--at temperatures ranging from about 600° C. to about 700° C.(preferably at atmospheric pressure) for from about 20 minutes to about60 minutes, and most preferably at about 650° C. for about 30 minutes.

TGA Measurements

Measurements of the absorption rate of SO_(X) on various experimentaladditives were obtained on a modified thermogravimetric analysis unit(TGA). The equipment consisted of a Polymer Laboratories STA 1500®thermogravimetric unit coupled with a microcomputer. Generally speaking,approximately 10 milligrams of a given sample was loaded into a ceramicsample boat and heated under various conditions. SO_(X) pick-up wasnormalized to the weight at the point where SO_(X) gas commenced to beintroduced. The composition of the SO₂ mix gas employed was usually 2000ppm SO₂, 5% CO₂, 1% O₂, with the balance being nitrogen. In the case oftests of SO₂ →SO₃ oxidants, mixtures (e.g., 50/50 mixtures by weight) ofan absorbent and an oxidant were used and the weight of the SO₃ pickedup the absorbent was measured. In general, when the objective of theexperiment was to test an SO₂ →SO₃ oxidant, a standard absorbent wasused; this standard absorbent was prepared by the process described inExample 1. In other experiments, unsupported magnesia powder wasemployed.

On the other hand, when the purpose of an experiment was to test anabsorbent, a standard oxidant, prepared as described in Example 7, wasused. The results of the TGA experiments were taken in two forms. Inone, the percent weight change after two hours of exposure to the SO₂gas mixture was measured. This result gave the maximum absorptioncapability of the mixture. In the second form of reporting the resultswas a calculation of the initial rate of absorption; these results wereexpressed as the % change in weight of the sample per minute. To theseends, applicants established certain criteria based on the TGA resultsthat must be met by a SO_(x) additive if it is to have commercialviability. For example, applicants have found that a SO_(x) additivemust have an initial pick-up rate of at least about 0.10 percent perminute in such tests, it also preferably will have a maximum absorptionof 40% by weight and lastly it must be regenerable. That is to say thatafter it reaches its maximum gain in weight it must rapidly return tothe base line when hydrogen is passed over the sample. Applicants havealso found that if a SO_(x) additive has a maximum absorption greaterthan 40%, this improvement is not always realized in commercialoperations. For example, applicants developed an additive that had amaximum pickup of 70% on the TGA; in commercial trials however thismaterial did not perform any better than an additive with a 50% maximumpickup. That is to say there was no corresponding reduction in theamount of additive required to remove a certain amount of SO_(x).

Comparisons with Certain Prior Art Single Particle Species, SO_(x)Additives

Applicants conducted various tests aimed at comparing themulti-particle, SO_(x) additive systems of this patent disclosure withvarious prior art, single particle, SO_(x) additives. These comparativetests were largely based upon comparative TGA tests. By way of exampleonly, the results of some of these tests are summarized in TABLE III.

                  TABLE III                                                       ______________________________________                                                                  Initial Rate                                                     Percent Increase in                                                                        of Absorption                                                    mass after 2 hours                                                                         (weight percent/                                    Composition  (weight percent)                                                                           minute)                                             ______________________________________                                        Single Particle                                                                            61           1.9                                                 SO.sub.x Additive                                                             Dual Particle                                                                              48           1.5                                                 Sample A*                                                                     Dual Particle                                                                              53           1.6                                                 Sample B*                                                                     Dual Particle                                                                              52           1.6                                                 Sample C*                                                                     ______________________________________                                         *Note that the following statements help to better describe the Dual          Particle samples:                                                             Sample A: Sample A was comprised of a mixture of absorbent particles from     Example 1 and oxidant particles from Example 7. The mixture contained 50      weight percent absorbent and 50 weight percent oxidant.                       Sample B: Sample B was comprised of a mixture of absorbent particles from     Example 2 and oxidant particles from Example 6. The mixture contained 50      weight percent absorbent and 50 weight percent oxidant.                       Sample C: Sample C was comprised of a mixture of absorbent particles from     Example 3 and oxidant particles from Example 6. The mixture contained 50      weight percent absorbent and 50 weight percent oxidant.                  

In all cases, the initial rate of absorption was determined for thefirst 15 minutes of SO_(x) pickup.

Other Findings Re: Comparative Tests

Applicants' comparative experimental program also established that manyhydrocarbon cracking catalysts commonly used in FCC units display some,albeit limited, ability to absorb SO₃. This is probably due to thewidespread use of certain active forms of alumina in many FCChydrocarbon cracking catalysts. Applicants believe that the limitationson the ability of these materials to absorb SO₃ generally follows fromthe fact that there are no SO₂ →SO₃ oxidant catalyst materials in suchhydrocarbon cracking catalyst particles. In other words, applicantsconcluded that many hydrocarbon cracking catalysts could also serve toabsorb SO₃ if a oxidation catalyst for converting SO₂ to SO₃ also wereused in conjunction with the hydrocarbon cracking catalysts. Applicantsalso found that this can be achieved even when at least one oxidationcatalyst is not physically associated with an SO₃ absorbent in the sameadditive particle and not physically associated with a hydrocarboncracking catalyst in the same particle. Several of applicants'experiments confirmed that all of this was indeed the case. Whenapplicants' SO₃ absorbent materials were in fact added to suchhydrocarbon cracking systems, it was found that a wide variety of FCChydrocarbon cracking catalyst (and especially bottoms cracking systems)also serve to absorb SO₃ while still performing their hydrocarboncracking function--if sufficient capability to catalyze the conversionof SO₂ to SO₃ exists in the overall catalyst system. Applicants alsofound that their SO_(x) additive systems can provide this "extra" SO₂ toSO₃ oxidation function particularly well when they are used in admixturewith hydrocarbon cracking catalysts and can, therefore, form the basisof particularly effective hydrocarbon cracking catalyst/SO_(x) additivesystems--and especially those comprised of a major amount (e.g.,90-99.5% by weight--on a dry weight basis) of a bulk, hydrocarboncracking catalyst and a minor amount (e.g., 0.5-10% by weight on a dryweight basis) of applicants' SO_(x) additive system.

Expressed in patent claim language such methods for extending the usefullife of a SO_(x) additive system having a SO₂ →SO₃ oxidation catalystcomponent and a SO₃ absorbent component will comprise: (1) employing theSO_(x) additive system in the form of at least two physically distinctparticle species wherein a first particle species contains the SO₂ →SO₃oxidation catalyst component and carries out a primary function ofoxidizing sulfur dioxide to sulfur trioxide and the second particlespecies is physically separate and distinct from the first particlespecies and carries out the function of absorbing the SO₃ produced bythe SO₂ →SO₃ oxidation catalyst component; (2) employing the SO₂ →SO₃oxidation catalyst component in the form of first particle thatcomprises: (a) a sulfur SO₂ →SO₃ oxidation catalyst comprised of a metalselected from the group consisting of a metal having an atomic number ofat least 20, a metal from Group 1B of the Periodic Table, a metal fromGroup 11B of the Periodic Table, a metal from Groups III and VIII of thePeriodic Table and a rare earth metal of the Periodic Table; and (b) abinder selected from the group of metal-containing compounds consistingof calcium aluminate, calcium silicate, aluminum titanate, zinctitanate, aluminum zirconate, magnesia, alumina, aluminum-containingmetal oxide compound, aluminum hydroxide, clay, zirconia, titania,silica, clay, clay/phosphate material and bastnaesite and which, ifemployed at all, contains no more than about 10 weight percent aluminumhydroxide and no more than about 10 weight percent alumina (Al₂ O₃); and(3) using the SO₃ absorbent component in the form of a second particlethat comprises: (a) a metal oxide selected from the group consisting ofcalcium aluminate, alumina, hydrotalcite, hydrotalcite-like compounds,magnesia and calcium oxide, and, as an optional ingredient, a supportmaterial selected from the group of metal oxides consisting of calciumaluminate, magnesia, alumina, aluminum nitrohydrate, aluminumchlorohydrate, silica, alumina, titania, kaolin clay, clay/phosphatematerial and zirconia, and which, if employed at all, contains no morethan about 10 weight percent silica and no more than about 50 weightpercent kaolin clay.

REPRESENTATIVE SO₃ ABSORBENT SYSTEMS Example 1

An alumina/magnesium hydroxy acetate/clay slurry was prepared by adding2495 grams of Condea P-3® Alumina Sol to 10.29 liters of watercontaining 111 grams of 84% concentrated acetic acid. The additions wereperformed under moderate agitation conditions. Thereafter, 2343 grams ofCondea P3® Alumina Sol, prepared in the manner noted above were added to2523 grams of magnesium hydroxy acetate. To the resulting slurry, 345grams of Theile RC-32® kaolin clay and 800 grams water were added. Theresulting slurry was spray dried and the particle products of the spraydrying were calcined at 650° C. for 30 minutes in a muffle furnace.

Example 2

An alumina/magnesium hydroxy acetate/vanadium oxalate slurry wasprepared by adding 2495 grams of Condea P-3® Alumina Sol to 10.29 litersof water that contained 111 grams of 84% concentrated acetic acid. Theseadditions were performed under moderate agitation conditions.Thereafter, 2524 grams of P3® alumina sol, prepared in the mannerdescribed above, was added to 2524 grams of magnesium hydroxy acetate.To the resulting slurry, 211 grams of vanadium oxalate and 800 grams ofwater were added. The slurry was then spray dried and the resultingparticles calcined at 650° C. for 30 minutes in a muffle furnace.

Example 3

An alumina/magnesium hydroxy acetate/clay/vanadium oxalate slurry wasprepared by adding, under moderate agitation, 2495 grams of Condea P-3®Alumina Sol to 10.29 liters of water containing 111 grams of 84%concentrated acetic acid. The additions were performed under moderateagitation. Thereafter, 2246 grams of Condea P3® Alumina Sol, prepared asnoted above, were added to 2419 grams of magnesium hydroxy acetate. Tothe resulting slurry, 345 grams of Theile RC-32® kaolin clay slurry, 211grams vanadium oxalate and 500 grams of water were added. The slurry wasspray dried and the resulting particles calcined at 650° C. for 30minutes in a muffle furnace.

Example 4

An alumina sol was first prepared by adding 2495 grams Condea P-3Alumina Sol to 10.29 liters water containing 111 grams 84% concentratedacetic acid. The additions were performed under moderate agitationconditions. Next, an aqueous slurry of magnesium silicate (R. T.Vanderbilt Ceramitalc No. 1) was prepared to produce a 37% solidsconcentration. The resulting slurry was reduced in particle size using acommercially available attritor mill. The duration of milling wasdetermined by the time required to obtain an average particle size of2-3 um. 2343 grams of Condea P3 alumina sol, prepared as noted above,was added to 2523 grams of magnesium hydroxy acetate. To this slurry,490 grams of the magnesium silicate slurry and 800 grams water wereadded. The completed slurry was spray dried and then calcined at 650° C.for 30 minutes in a muffle furnace.

Example 5

2501 grams of P3 alumina sol was added to 2023 grams of magnesiumhydroxy acetate. To this slurry, 409 grams of cerium nitrate was added;this was followed by the addition of 213 grams of vanadium oxalate and382 grams of Theile RC-32 kaolin clay slurry. The slurry was spray driedand then calcined at 650° C. for 30 in a muffle furnace.

Representative SO₂ →SO₃ Oxidant/Binder Systems Example 6

2092 grams of P3 alumina sol was added to 1692 grams of magnesiumhydroxy acetate. To this slurry, 897 grams of cerium nitrate was added;this was followed by the addition of 382 grams of Theile RC-32 kaolinclay slurry to the system. The slurry was spray dried and then calcinedat 650° C. for 30 minutes in a muffle furnace.

Example 7

1864 grams of P3 alumina sol was added to 1508 grams of magnesiumhydroxy acetate. To this slurry, 897 grams of cerium nitrate was added;this was followed by the addition of 448 grams of vanadium oxalate and382 grams of Theile RC-32 kaolin clay slurry. The slurry was spray driedand then calcined at 650° C. for 30 minutes in a muffle furnace.

Example 8

A commercially available oxidation catalyst sold under the trade nameCOP 850 was used as an oxidant. This material consists of 850 ppm ofplatinum impregnated on a substrate of alpha alumina.

Examples 9-13

The following SO_(x) pick up test results were produced by mixtures ofabsorbent particles and oxidant particles. The ThermogravimetricAnalyzer previously described was used as the SO_(x) pick up measuringinstrument. The SO_(x) absorbence results of the various mixtures werecompared to those produced by a single particle. These measurements aresummarized in the following Table IV:

                  TABLE IV                                                        ______________________________________                                        Test Sample                  Increase in                                                                           Initial rate of                          Made According               mass after 2                                                                          Absorption,                              to Example                                                                              Absorbent                                                                              Oxidant   hours, % wt.                                                                          % wt/min.                                ______________________________________                                         5        Single Particle                                                                              63        1.9                                         9        50% Ex. 1                                                                              50% Ex. 7 51      1.8                                      10        50% Ex. 2                                                                              50% Ex. 6 57      1.7                                      11        50% Ex. 3                                                                              50% Ex. 6 53      1.7                                      12        50% Ex. 4                                                                              50% Ex. 6 55      1.7                                      13        75% Ex. 2                                                                              25% Ex. 8 55      2.0                                      ______________________________________                                    

Representative SO₂ →SO₃ Oxidant in Clay-Phosphate Binder Systems Example14

A clay/phosphate/cerium nitrate solution was prepared (according to thegeneral teachings of U.S. Pat. No. 5,190,902) by adding 1403 grams ofTheile RC-32® kaolin clay slurry to 591 milliliters of water in a highspeed mixer. To this mixture, 192 grams of phosphoric acid was thenadded. To the resulting system, 440 grams of cerium nitrate solution wasadded. The slurry formulation was then spray dried and the resultingparticles calcined at 650° C. for 30 minutes in a muffle furnace. Thecalcined particles were then mixed with magnesium oxide in a 50/50weight percent ratio and the resulting material analyzed by a TGA testthat showed a SO_(x) absorption rate of 0.21% per minute for the endproduct material.

Example 15

A clay/phosphate/cerium nitrate, vanadium oxalate slurry was prepared byadding 1340 grams of Theile RC-32® kaolin clay slurry to 1795milliliters of water in a high speed mixer. To this mixture, 192 gramsof phosphoric acid was added. Thereafter, 431 grams of cerium nitratesolution and 232 grams of vanadium oxalate solution were added to theslurry. The slurry was then spray dried and then resulting particleswere calcined at 650° C. for 30 minutes in a muffle furnace. Thecalcined particles were then mixed with magnesium oxide in a 50/50weight percent ratio and analyzed by TGA tests. These tests indicatedthat the resulting material produced an SO_(x) absorption of 0.27% perminute.

Example 16

A clay/phosphate/cerium carbonate slurry, was prepared by adding 1403grams of Theile RC-32® kaolin clay slurry to 1200 milliliters of waterin a high speed mixer. To this mixture, 192 grams of phosphoric acid wasadded; thereafter 277 grams of cerium carbonate were added. Thecompleted slurry was spray dried and the resulting particles calcined at650° C. for 30 minutes in a muffle furnace. The calcined material wasthen mixed with magnesium oxide in a 50/50 weight percent ratio andanalyzed by the TGA tests, which showed a SO_(X) absorption of 0.15% perminute.

Collectively, Examples 9-13 show that a physical mixture of an absorbent(Examples 1-4) and an oxidant (Examples 6-8) give the same SO_(x)removal as a single particle (Example 5). Example 9 is a mixture ofalumina/magnesia absorbent with kaolin clay used as a hardening agentmixed with an oxidant containing ceria and vanadia (Example 7). Example10 is a mixture of an absorbent (made according to Example 2) in whichone of the oxidants, in this case vanadia, is included with theabsorbent; the other oxidant, ceria, is in the particle in admixturewith the absorbent (made according to Example 6). The significance ofthis experiment lies in the fact that while vanadia is a minor componentin the additive system it is considered to be a catalyst "poison;"hence, including it with the magnesia/alumina insures that it isimmobilized and thus cannot be transferred by sublimation from theSO_(x) additive mixture to the host catalyst. Example 11 is identical toExample 10 with the exception that kaolin clay is used as a hardeningagent in a magnesia/alumina/vanadia absorbent. Example 12 is the same asExample 9 except that magnesium silicate is used as the hardening agentinstead of clay. In Example 13 a commercially available oxidationcatalyst consisting of platinum on alumina is used in place of the ceriaoxidant. The significance of this example is that many FCC units arealready using a platinum promoter to convert carbon monoxide to carbondioxide and this same material may also serve to convert sulfur dioxideto sulfur trioxide and thus the need for a ceria oxidant can be greatlyreduced or eliminated entirely.

Example 17

A calcium aluminate support for a SO₂ →SO₃ oxidation catalyst componentwas prepared by first making an alumina sol consisting of adding 2495grams of Condea P-3 Alumina Sol and 10.29 liters of water containing 111grams of 84% concentrated acetic acid. In a separate container, undermoderate agitation conditions, 581.8 grams of Huber calcium carbonatewas added to 2 liters of water. To this mixture, 4876.9 grams of thepreviously prepared alumina sol was added. The resulting slurry wasspray dried and the particle products of the spray drying were calcinedat 650° C. for 30 minutes in a muffle furnace.

Example 18

An SO₃ absorbent component containing hydrotalcite was prepared by firstpreparing a gel of alumina consisting of 189.0 grams of Condea SBAlumina, 28.3 grams of 74% concentrated formic acid and 917 ml of water.In a separate container, under moderate agitation conditions, 808.2grams of LaRoche hydrotalcite was added to 1147 milliliters of water. Tothis mixture, 1512 grams of the previously prepared alumina gel wasadded. Thereafter, 362.1 grams of Theile RC-32 kaolin clay slurry wasadded to the resulting slurry. The resulting slurry was spray dried andthe particle products of the spray drying were calcined at 650° C. for30 minutes in a muffle furnace.

Calcium aluminate is a particularly effective material for the practiceof this invention in that it is capable of serving both as a binder (orsupport) for the SO₂ →SO₃ oxidation catalyst component of the SO_(x)additive system and as a SO₃ absorbent component as well. In otherwords, calcium aluminate is a binder material that also has SO₃absorbent capabilities as well. Hence, it can serve especially well as abinder for other SO₃ absorbent materials such as hydrotalcite that areused to make the SO₃ absorbent component of the SO_(x) additive system.

While this invention has been described with respect to various specificexamples and a spirit which is committed to the concept of the use ofmulti-particle SO_(x) additive systems, it is to be understood that thehereindescribed invention should only be limited by the scope of thefollowing claims.

Thus, what is claimed is:
 1. A fluid SO_(x) additive system comprisingSO₂ →SO₃ oxidation catalyst particles and SO₃ absorbent particleswherein:(1) the SO₂ →SO₃ oxidation catalyst particles comprise: (a) aSO₂ →SO₃ oxidation catalyst comprised of a metal selected from the groupconsisting of cerium, vanadium, platinum, palladium, rhodium,molybdenum, tungsten, copper, chromium, nickel, iridium, manganese,cobalt, iron, ytterbium, and uranium; and (b) a binder made from amaterial selected from the group of metal-containing compoundsconsisting of calcium aluminate, aluminum silicate, aluminum titanate,zinc titanate, aluminum zirconate, magnesium aluminate, magnesia,alumina (Al₂ O₃), aluminum hydroxide, an aluminum-containing metal oxidecompound (other than alumina (Al₂ O₃)), clay, zirconia, titania, silica,clay/phosphate, bastnaesite and which contains no more than about 10weight percent aluminum hydroxide and no more than about 10 weightpercent alumina (Al₂ O₃); and (2) the SO₃ absorbent particles arephysically separate and distinct from the SO₂ →SO₃ oxidation catalystparticles and comprise a metal oxide selected from the group consistingof hydrotalcite, hydrotalcite-like compositions, magnesia, magnesiumacetate, magnesium nitrate, magnesium chloride, magnesium hydroxide,magnesium carbonate, magnesium formate, magnesium aluminate, hydrousmagnesium silicate, magnesium calcium silicate, calcium silicate,alumina, calcium oxide and calcium aluminate.
 2. The fluid SO_(x)additive system of claim 1 wherein the SO₂ →SO₃ oxidation catalystparticles are comprised of ceria.
 3. The fluid SO_(x) additive system ofclaim 1 wherein the SO₂ →SO₃ oxidation catalyst particles are comprisedof vanadia.
 4. The fluid SO_(x) additive system of claim 1 wherein theSO₂ →SO₃ oxidation catalyst particles are comprised of ceria andvanadia.
 5. The fluid SO_(x) additive system of claim 1 wherein thebinder for the SO₂ →SO₃ oxidation catalyst particles are comprised ofcalcium aluminate.
 6. The fluid SO_(x) additive system of claim 1wherein the oxidation catalyst particles contain no aluminum hydroxideand no alumina (Al₂ O₃).
 7. The fluid SO_(x) additive system of claim 1wherein the SO₃ absorbent particles further comprise a hardening agent.8. The fluid SO_(x) additive system of claim 1 wherein the SO₃ absorbentparticles further comprise a hardening agent selected from the groupconsisting of aluminum silicate, magnesium aluminate, magnesiumsilicate, magnesium calcium silicate and sepiolite.
 9. The fluid SO_(x)additive system of claim 1 wherein the SO₃ absorbent particles furthercomprise a SO₂ →SO₃ oxidation catalyst that is used in addition to theSO₂ →SO₃ oxidation catalyst in the SO₂ →SO₃ oxidation catalystparticles.
 10. The fluid SO_(x) additive system of claim 1 wherein theSO₃ absorbent particles further comprise a SO₂ →SO₃ oxidation catalystselected from the group consisting of cerium, vanadium, platinum,palladium, rhodium, iridium, molybdenum, tungsten, copper, chromium,nickel, manganese, cobalt, iron, ytterbium, and uranium.
 11. The fluidSO_(x) additive system of claim 1 wherein the SO₃ absorbent particlesfurther comprise a support material made from a metal-containingcompound selected from the group consisting of calcium aluminate,aluminum nitrohydrate, aluminum chlorohydrate, magnesia, silica,silicon-containing compounds (other than silica), alumina, titania,zirconia, clay and a clay phosphate material.
 12. The fluid SO_(x)additive system of claim 1 wherein the SO₃ absorbent particles furthercomprise a SO₂ →SO₃ oxidation catalyst that has a vanadia component anda SO₃ absorbent component that has a magnesia component.
 13. The fluidSO_(x) additive system of claim 1 wherein the SO₃ absorbent particlesare comprised of hydrotalcite.
 14. The fluid SO_(x) additive system ofclaim 1 wherein a binder for the SO₂ →SO₃ oxidation catalyst particlesare comprised of calcium aluminate and the SO₃ absorbent particles arecomprised of hydrotalcite.
 15. The fluid SO_(x) additive system of claim1 wherein a binder for the SO₂ →SO₃ oxidation catalyst particles arecomprised of calcium aluminate and the SO₃ absorbent particles arecomprised of calcium aluminate and hydrotalcite.
 16. The fluid SO_(x)additive system of claim 1 wherein the SO₃ absorbent particles arecomprised of a hydrotalcite-like material.
 17. The fluid SO_(x) additivesystem of claim 1 wherein a binder for the SO₂ →SO₃ oxidation catalystparticles are comprised of calcium aluminate and the SO₃ absorbentparticles are comprised of a hydrotalcite-like material.
 18. The fluidSO_(x) additive system of claim 1 wherein a binder for the SO₂ →SO₃oxidation catalyst particles are comprised of calcium aluminate and theSO₃ absorbent particles are comprised of calcium aluminate and ahydrotalcite-like material.
 19. The fluid SO_(x) additive of claim 1wherein the SO₂ →SO₃ oxidation catalyst particles are themselvescomprised of at least two separate and distinct particles species. 20.The fluid SO_(x) additive system of claim 1 wherein the SO₃ absorbentparticles are themselves comprised of at least two separate and distinctparticle species.
 21. The fluid SO_(x) additive system of claim 1wherein the SO₂ →SO₃ oxidation catalyst particles and the SO₃ absorbentparticles are each microspheroidal particles suitable for circulation inan FCC unit in admixture with at least one microspheroidal particlespecies whose primary function is to catalytically crack a hydrocarbonfeedstock.
 22. The fluid SO_(x) additive system of claim 1 wherein theSO₂ →SO₃ oxidation catalyst particles and the SO₃ absorbent particlesare each pellets.
 23. The fluid SO_(x) additive system of claim 1wherein the SO₂ →SO₃ oxidation catalyst particles and the SO₃ absorbentparticles are each pellets of a size suitable for use in a moving bedcatalyst system.
 24. The fluid SO_(x) additive system of claim 1 whereinthe SO₂ →SO₃ oxidation catalyst particles comprise from about 10 toabout 90 weight percent of said fluid SO_(x) additive system.
 25. Thefluid SO_(x) additive system of claim 1 wherein the fluid SO_(x)additive system comprises from about 0.5 to about 10.0 weight percent ofa bulk hydrocarbon cracking catalyst/SO_(x) additive system.